On Fri, Nov 4, 2022 at 12:02 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Nov 2, 2022 at 8:35 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > Resolve forward declarations that don't take part in type graphs
> > comparisons if declaration name is unambiguous. Example:
> >
> > CU #1:
> >
> > struct foo; // standalone forward declaration
> > struct foo *some_global;
> >
> > CU #2:
> >
> > struct foo { int x; };
> > struct foo *another_global;
> >
> > The `struct foo` from CU #1 is not a part of any definition that is
> > compared against another definition while `btf_dedup_struct_types`
> > processes structural types. The the BTF after `btf_dedup_struct_types`
> > the BTF looks as follows:
> >
> > [1] STRUCT 'foo' size=4 vlen=1 ...
> > [2] INT 'int' size=4 ...
> > [3] PTR '(anon)' type_id=1
> > [4] FWD 'foo' fwd_kind=struct
> > [5] PTR '(anon)' type_id=4
> >
> > This commit adds a new pass `btf_dedup_resolve_fwds`, that maps such
> > forward declarations to structs or unions with identical name in case
> > if the name is not ambiguous.
> >
> > The pass is positioned before `btf_dedup_ref_types` so that types
> > [3] and [5] could be merged as a same type after [1] and [4] are merged.
> > The final result for the example above looks as follows:
> >
> > [1] STRUCT 'foo' size=4 vlen=1
> > 'x' type_id=2 bits_offset=0
> > [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> > [3] PTR '(anon)' type_id=1
> >
> > For defconfig kernel with BTF enabled this removes 63 forward
> > declarations. Examples of removed declarations: `pt_regs`, `in6_addr`.
> > The running time of `btf__dedup` function is increased by about 3%.
> >
> > Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> > Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > ---
> > tools/lib/bpf/btf.c | 143 ++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 139 insertions(+), 4 deletions(-)
> >
>
> LGTM, small nit about hashmap__new initialization
>
> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
>
>
> > +}
> > +
> > +static int btf_dedup_resolve_fwd(struct btf_dedup *d, struct hashmap *names_map, __u32 type_id)
> > +{
> > + struct btf_type *t = btf_type_by_id(d->btf, type_id);
> > + enum btf_fwd_kind fwd_kind = btf_kflag(t);
>
> this is a bit subtle, but probably won't ever break as enum
> btf_fwd_kind is part of libbpf UAPI
>
> > + __u16 cand_kind, kind = btf_kind(t);
> > + struct btf_type *cand_t;
> > + uintptr_t cand_id;
> > +
> > + if (kind != BTF_KIND_FWD)
> > + return 0;
> > +
> > + /* Skip if this FWD already has a mapping */
> > + if (type_id != d->map[type_id])
> > + return 0;
> > +
> > + if (!hashmap__find(names_map, t->name_off, &cand_id))
> > + return 0;
> > +
> > + /* Zero is a special value indicating that name is not unique */
> > + if (!cand_id)
> > + return 0;
> > +
> > + cand_t = btf_type_by_id(d->btf, cand_id);
> > + cand_kind = btf_kind(cand_t);
> > + if ((cand_kind == BTF_KIND_STRUCT && fwd_kind != BTF_FWD_STRUCT) ||
> > + (cand_kind == BTF_KIND_UNION && fwd_kind != BTF_FWD_UNION))
> > + return 0;
> > +
> > + d->map[type_id] = cand_id;
> > +
> > + return 0;
> > +}
> > +
> > +/*
> > + * Resolve unambiguous forward declarations.
> > + *
> > + * The lion's share of all FWD declarations is resolved during
> > + * `btf_dedup_struct_types` phase when different type graphs are
> > + * compared against each other. However, if in some compilation unit a
> > + * FWD declaration is not a part of a type graph compared against
> > + * another type graph that declaration's canonical type would not be
> > + * changed. Example:
> > + *
> > + * CU #1:
> > + *
> > + * struct foo;
> > + * struct foo *some_global;
> > + *
> > + * CU #2:
> > + *
> > + * struct foo { int u; };
> > + * struct foo *another_global;
> > + *
> > + * After `btf_dedup_struct_types` the BTF looks as follows:
> > + *
> > + * [1] STRUCT 'foo' size=4 vlen=1 ...
> > + * [2] INT 'int' size=4 ...
> > + * [3] PTR '(anon)' type_id=1
> > + * [4] FWD 'foo' fwd_kind=struct
> > + * [5] PTR '(anon)' type_id=4
> > + *
> > + * This pass assumes that such FWD declarations should be mapped to
> > + * structs or unions with identical name in case if the name is not
> > + * ambiguous.
> > + */
> > +static int btf_dedup_resolve_fwds(struct btf_dedup *d)
> > +{
> > + int i, err;
> > + struct hashmap *names_map =
> > + hashmap__new(btf_dedup_identity_hash_fn, btf_dedup_equal_fn, NULL);
>
> if variable declaration and initialization doesn't even fit in a
> single line, that's a signal that they should better be split.
>
> In general we also try to avoid doing "complex" initialization at
> declaration time. So please split.
>
> > +
> > + if (!names_map)
also:
if (IS_ERR(names_map))
return PTR_ERR(names_map);
> > + return -ENOMEM;
> > +
>
> [...]
